Method of direct reduction of iron ore

ABSTRACT

The stack gas from a shaft furnace in which iron ore is directly reduced is cooled and filtered and then subjected to steam so as to convert its carbon monoxide into carbon dioxide and water. The carbon dioxide and water are then scrubbed out of this stack gas so as to leave virtually pure hydrogen which is heated to a temperature between 400° and 600° C and added to the output of an oxygen reactor that transforms coal and oxygen into a synthetic gas having a carbon-monoxide/hydrogen volume ratio of at least 1.5:1. The addition of this hydrogen to the output of the reactor lowers the volume ratio to 1:1 at most so that this mixture can be reintroduced into the shaft furnace at a temperature of around 1000° C as a reducing gas.

FIELD OF THE INVENTION

The present invention relates to the direct reduction of iron ore. Moreparticularly this invention concerns the reduction of iron ore pelletsinto sponge iron using a reducing gas comprised principally of carbonmonoxide and hydrogen.

BACKGROUND OF THE INVENTION

It is known to directly reduce iron ore by use of a reducing gas formedthrough the gasification of hydrocarbon. Natural gas is conventionallyconverted thermally or catalytically, if necessary with the addition ofsteam and/or oxygen, to a reducing gas formed principally of carbonmonoxide and hydrogen, with the ratio between these two components being1:1 or less. This reducing gas is then fed substantially at thetemperature it attains in the conversion process into the shaft furnacewhere the direction reduction takes place. A portion of the stack gasproduced by direct reduction is fed back to the converter as anoxygen-carrier where its carbon monoxide and hydrogen content isexploited. Another portion of the stack gas is used for simple heatexchange.

The principal difficulty with such a system is that it must employrelatively expensive basic fuels for the production of the originalreducing gas. Thus natural gas or other light hydrocarbons must beemployed.

It is known to gasify coal with oxygen to produce a mixture of carbonmonoxide and hydrogen. See for example "Vergasung fester Brennstoffe undOxydative Unwandlung von Kohlenwasserstoffen" by Jacques Meunier (VerlagChemie GmbH, Weinheim/Bergstrasse: 1962 ).

The synthetic-gas mixture so produced has a carbon monoxide/hydrogenratio of at least 1.5:1. Use of this gas as a reduction gas produces avery wet stack gas having a carbon monoxide or carbon dioxide tohydrogen and water ratio of approximately 1.5:1. Filtering out of thewater and dust in this stack gas by heating it up again to 1000° C.causes the carbon monoxide to decay into soot and carbon dioxide, makingthe operation extremely messy and, therefore, costly.

OBJECTS OF THE INVENTION

It is therefore an object of the present invention to provide animproved direct-reduction method.

It is another object of this invention to provide such a method whichallows a relatively cheap fuel such as coal to be employed for theproduction of the basic reducing gas, and which at the same time is ableto recirculate and employ any usable constituents of the stack gas.

SUMMARY OF THE INVENTION

These objects are attained according to the present invention in asystem wherein coal and oxygen are reacted to make a producer gas formedprincipally of carbon monoxide and hydrogen in a volume ratio of atleast 1.5:1. The stack gas is treated with steam to convert it into amixture of carbon dioxide and hydrogen and then this carbon dioxide isscrubbed out of the stack gas leaving relatively pure hydrogen. Thishydrogen is added to the producer gas produced by the gasification ofcoal so as to form a reducing gas having a carbon monoxide/hydrogenvolume ratio of at most C. which gas is usuable for the reduction ofiron ore.

In accordance with the present invention the stack gas as it issues fromthe shaft furnace in which the iron ore is reduced is first passedthrough a venturi scrubber and then through a heater where itstemperature is raised to around 400° C. Thereafter superheated steam isadded to the gas for water-gas shift reaction that reduces the carbonmonoxide in the gas to carbon dioxide. Another scrubbing strips thecarbon dioxide and water from the gas, leaving almost pure hydrogenwhich is heated to a temperature of between 400° and 600 ° C and addedto the very hot gases issuing from the oxygen reactor where the coal isgasified.

In the oxygen reactor according to this invention a sufficient quantityof oxygen is reacted with the elemental coal so as to produce a very hotgas mixture, above 1300° C., to which is added the relatively purehydrogen coming from the stack-gas recirculator. This entire mixture iscycloned to remove any particular impurities and then pass through anadsorber or stripper to remove sulfur from it. The resulting reducinggas is at a temperature of approximately 100° C. and has approximatelyequal parts by weight of carbon monoxide and hydrogen.

With the system according to the present invention it is possible to usea very cheap starting fuel such as coal or crude oil, but to enrich thesynthetic gas produced therefrom by the addition thereto of hydrogenrecovered from the stack gas. In this manner decoy of the carbonmonoxide into soot is almost completely eliminated.

BRIEF DESCRIPTION OF THE DRAWING

The above and other objects, features, and advantages of the inventionwill become more readily apparent from the following description,reference being made to the drawing whose sole FIGURE is a schematicdiagram of a system for carrying out the accompanying method accordingto the present invention.

SPECIFIC DESCRIPTION

A shaft furnace 1 is fed with iron ore and reducing gas and produces astack gas comprising mainly carbon monoxide, water, hydrogen, and carbondioxide, along with various particulate impurities. The reducing gas isproduced initially by an oxygen reactor 2 and the stack gas isrecirculated through an arrangement 3 described below.

Part of the stack gas is not recirculated but is used as indicated byarrow 9 for heat exchange and other purposes. The balance of the stackgas is fed to a venturi scrubber 8 where it is stripped of particulatematerial and its temperature is dropped considerably. From this scrubber8 the gas is passed through a heater 10 where its temperature is raisedto approximately 400° C. Thereafter the particle-free stack gas, stillcomprising basically carbon monoxide, water, hydrogen, and carbondioxide, is fed to a carbon monoxide converter 4 to which steam is fedas indicated at 12. In this converter 4 a so-called water-gas shiftreaction takes place, that is the carbon monoxide and water in the gasare transformed principally into carbon dioxide and hydrogen, thus C0+H₂0→CO₂ +H₂.

After exiting from the scrubber 4 the gas, which is now free of carbonmonoxide, is passed through another scrubber 5 where the water isstripped from the gas and the carbon dioxide is absorbed. This leavesvirtually pure hydrogen which is then heated by means of a heater 11 toapproximately 500° C. and fed to a line 13.

In the oxygen reactor 2 coal is reacted with oxygen much as described onpage 549 ff of Chemical and Process Encyclopedia by D. M. Considine(McGraw Hill: 1974). At the output side 14 of this reactor 2 a gasmixture is produced having a temperature of approximately 1600° C. andcomprised basically of carbon monoxide and hydrogen in a ratio of 1.5:1,with various particulate impurities and sulfur. The line 13 carrying the500° C. hydrogen from the heater 11 is connected to the line 14 to forma mixture that is fed to a cyclone 6 where all of the particulateimpurities are removed from the gas. Thereafter the particle-free gas isstripped of its sulfur by an adsorber 7 of the type using dolomite.Finally the output 15 of the adsorber 7 is connected to the reducing-gasinput 16 of the shaft furnace 1 so as to feed to this input 16 a mixtureof carbon monoxide and hydrogen in a ratio of at most 1:1 by weight andat a temperature of approximately 1000° C.

I claim:
 1. In the reduction of iron ore wherein the ore is subjected toa reducing gas and a stack gas is produced, the improvement comprisingthe steps of:a. reacting coal and oxygen to make a producer gas ofcarbon monoxide and hydrogen in a volume ratio of at least 1.5:1 andsulfur and at a temperature of about 1600° C.; b. treating said stackgas with steam to convert said stack gas into a converter-gas mixture ofcarbon dioxide and hydrogen; c. scrubbing said carbon dioxide out of theconverter-gas mixture of step (b) to form a hydrogen-containing gas; d.heating said hydrogen-containing gas to a temperature of about 400° C.;e. adding the producer gas of step (a) to the hydrogen-containing gas ofstep (d) to form a reducing gas having a volume ratio of carbon monoxideto hydrogen of at most 1:1 f. stripping sulfur from the gas produced instep (e); g. reducing the iron ore with the reduction gas of step (f).2. The improvement defined in claim 1, further comprising the step ofcooling said stack gas to between 400° C. and 600° C. prior to treatingsame with steam.